In recent years, a multirotor flight vehicle, which includes four or more propellers, each having two or three blades, and flies by rotating them, has begun to be frequently used for the purpose of transportation and photographing. The multirotor flight vehicle flies by rotating these propellers, and hence generates airfoil flow noise. In addition, as the sizes of flight vehicles increase, noise increasingly raises problems.
As a method of reducing noise, ANC (active noise control) is known, ANC outputs a signal (control sound) having the same amplitude and an opposite phase as compared with noise from a control loudspeaker in order to reduce noise.
Honda, Y., Saburi, S., Matsuhisa, H, and Sato, S., Active Minimization of Blade Rotational Noise from an Axial Fan, The Japan Society of Mechanical Engineers, (C)59(562), 228-233 (1933), which presents theoretical analysis on an active noise reduction technique for axial fan noise, discloses a rotating blade rotation model of an axial fan as a multiple sound source and a method of reducing fan noise by arranging control loudspeakers coaxially in a circumferential form with respect to a rotating blade rotation center so as to minimize acoustic power. In addition, according to Aoki, T., Morishita, T., Tanaka, T. and Taki, M., Active control of fan noise in a free field based on the spherical harmonic expansion, Acoustical Society of Japan, 59(2003) 379-387 and Aoki, T., Tanaka, T. and Taki, M., Active control of circular noise source using discrete ring sources: A theoretical consideration, Acoustical Society of Japan, 60(2004) 639-645, a rotating blade rotation model is expanded based on spherical surface harmonics, and mode amplitudes from a main source originating from rotating blade rotation and a control sound source originating from control loudspeakers arranged on a circumference are matched with each other to implement active noise reduction. According to the three literatures described above, the count of control loudspeakers arranged on a circumference has been derived to be 2M+1 as a minimum count and 2M+3 (even using 2M+3 or more control loudspeakers will not reduce the effect) as a maximum count.